A letter addressed to US president Barack Obama containing a substance that initially tested positive for the poison ricin was intercepted a day after a similar letter was sent to a senator.
The poison letter heightened fears and intensified security efforts in an already febrile atmosphere following the Boston Marathon bombs on Monday that killed three people and injured more than 170.
The FBI said that the letter, intercepted at a remote facility where the president’s post is checked, tested positive for ricin in a preliminary screening but the law enforcement bureau stressed that there was no connection with the Boston blasts.
Further analysis
Investigators said that the contents of the letter were being sent to a laboratory for further analysis and that the results would be expected in 24 to 48 hours.
A letter suspected of containing ricin, addressed to Senator Roger Wicker, a Republican from Mississippi, was discovered on Tuesday at another facility that checks post destined for members of Congress.
White House press spokesman Jay Carney said that the FBI had not made any connection with the bomb blasts in Boston.
“The FBI has the lead in that investigation, of course, and has said in its statement [that] they will be conducting further tests to determine what the nature of the substance is,” he said.
Another senator, Richard Shelby, a Republican from Alabama, was also drawn into the investigation after it was reported that he had received a similarly suspicious letter. The letter to Mr Wicker carried a postmark showing that it was sent from Memphis, Tennessee, and it was dated April 8th.
Investigators have reportedly identified a suspect in the Wicker letter. The individual was said to be someone who "writes to a lot of members [of Congress]," Senator Claire McCaskill, a Democrat from Missouri told reporters.
Police on Capitol Hill quarantined areas of the Hart and Russell buildings, part of the Senate complex, in Washington as part of an investigation into the contaminated post involving the Secret Service and the FBI.
Exposure to ricin can induce various symptoms ranging from nausea, vomiting and gastric haemorrhaging to death within three to five days if the dose is sufficiently strong.
The tainted letters are similar to contaminated mail sent a week after the September 2001 attacks to members of Congress that contained anthrax spores. Five people died after touching the letters.
The latest scare in Washington came as investigators said they had found video footage of a man they believe may have planted the bombs at the Boston Marathon.
The footage was captured on camera shortly before one of the blasts.
The video evidence was the first breakthrough in the 48-hour investigation into the bombings that had until then identified no suspects or motives for the attacks that have left Bostonians shattered.
Surveillance cameras
Scores of videos and photographs taken of Monday's high-profile event, attended by an estimated 500,000 spectators, were being examined by law-enforcement officials after they called for images from surveillance cameras in nearby businesses, cameras and phones of spectators, and television crews.
Forensic investigators continued to comb the site of the two Boston bomb blasts on Boylston Street in the Back Bay area of the city near the finish line of the world’s oldest marathon.
Security remained tight as armed police and National Guard members policed a locked-down area around the blast sites.
Marathon runners, visible in their prominent blue-and-yellow Boston Marathon sweatshirts and jackets, continued to revisit the area to pay their respects to the dead.
Flowers and posters of support, including one encouraging Boston to “keep on running”, were left at security cordons blocking off access to the explosion sites.
